The Origin of the Solar System

Interstellar Cycle Ultimately, stars form the interstellar medium. Stars replenish the interstellar medium at the end of their life cycle. There is a balance between the interstellar medium and stars.

The Interstellar Medium 99% gas Mostly Hydrogen and Helium 1% dust Some volatile molecules H20, CO2, CO, CH4, NH3 Most common Metals Graphites Silicates (Fe, Al, Mg) (C) (Si)

Molecules Found Based on spectroscopy, more than 136 molecules have been found in the ISM. Among them the most common, H2 and the more exotic like formaldehyde and glycine and acetic acid.

Interstellar Molecules Here are some formaldehyde (H2CO) emission spectra from different parts of M20:

How Did the Solar System Form? Nebular contraction: Cloud of gas and dust contracts due to gravity; conservation of angular momentum means it spins faster and faster as it contracts

How Did the Solar System Form? Conservation of angular momentum says that product of radius and rotation rate must be constant:

Solar System Formation

How Did the Solar System Form? Condensation theory: Interstellar dust grains help cool cloud, and act as condensation nuclei

Asteroid Belt Ice Line Kuiper Belt T  T Terrestrial planets Jovian planets Condensation – where things become solid Where could the dust be solid? Where could the ices be solid? Where would you get terrestrial planets? Jovian planets?

The Condensation of Solids To compare densities of planets, compensate for compression due to the planet’s gravity: Only condensed materials could stick together to form planets Temperature in the protostellar cloud decreased outward. Further out  Protostellar cloud cooler  metals with lower melting point condensed  change of chemical composition throughout solar system

How Did the Solar System Form? Temperature in cloud determines where various materials condense out:

Formation and Growth of Planetesimals Planet formation starts with clumping together of grains of solid matter: Planetesimals Planetesimals (few cm to km in size) collide to form planets. Planetesimal growth through condensation and accretion. Gravitational instabilities may have helped in the growth of planetesimals into protoplanets.

The Story of Planet Building Planets formed from the same protostellar material as the sun, still found in the Sun’s atmosphere. Rocky planet material formed from clumping together of dust grains in the protostellar cloud. Mass of less than ~ 15 Earth masses: Mass of more than ~ 15 Earth masses: Planets can grow by gravitationally attracting material from the protostellar cloud Planets can not grow by gravitational collapse Earthlike planets Jovian planets (gas giants)

The Growth of Protoplanets Simplest form of planet growth: Unchanged composition of accreted matter over time As rocks melted, heavier elements sink to the center  differentiation This also produces a secondary atmosphere  outgassing Improvement of this scenario: Gradual change of grain composition due to cooling of nebula and storing of heat from potential energy

Primary Atmospheres The primary atmosphere for every terrestrial world was composed mostly of light gases that accreted during initial formation. These gases are similar to the primordial mixture of gases found in the Sun and Jupiter. That is 94.2% H, 5.7% He and everything else less that 0.1%. This primary atmosphere was lost on the terrestrial planets. Why? mass, radius of planet (factors of escape velocity of a planet) surface temperature (distance from Sun plus effects of atmosphere heating) Mass of the atoms What determines if a particular atom is retained by a planet's gravitational field? if the atom is moving less than the escape velocity for the planet, it stays. If it moves faster than escape velocity, it escapes into outer space. So note that for the outer Jovian worlds, all the primary, initial atmosphere is held. But for the inner worlds, most of the original H and He has been lost. These inner worlds then will form a secondary atmosphere composed of the outgassing from tectonic activity.

Secondary Atmospheres For the warmer terrestrial worlds, the light, gaseous elements (H, He) are lost. The remaining elements are grouped into the rocky materials (iron, olivine, pyroxene) and the icy materials (H2O, CO2, CH4, NH3, SO2). The icy materials are more common in the outer Solar System, they are delivered to the inner Solar System in the form of comets. The rocky and icy materials mix in the early crust and mantle. If the planet cools quickly, there is little to no tectonic activity and the icy materials are trapped in the mantle (like the Galilean moons). If the planet has a large mass (which means lots of trapped heat from formation), then there is a large amount of tectonic activity -> volcanos. The icy materials are turned to gases in the warm mantle and returned to the planet surface in the form of outgassing to produce a secondary atmosphere. The atmospheres of Venus, Earth and Mars are secondary atmospheres. The composition of outgassing is similar for Venus, Earth and Mars and is composed of 58% H2O, 23% CO2, 13% SO2, 5% N2 and traces of noble gases (Ne, Ar, Kr). The latter evolution of this outgassing is driven primarily by the surface temperature and chemistry of the planet.

The Jovian Problem Two problems for the theory of planet formation: 1) Observations of extrasolar planets indicate that Jovian planets are common. 2) Protoplanetary disks tend to be evaporated quickly (typically within ~ 100,000 years) by the radiation of nearby massive stars.  Too short for Jovian planets to grow! Solution: Computer simulations show that Jovian planets can grow by direct gas accretion without forming rocky planetesimals.

Our Solar System

All orbits but Pluto’s are close to same plane The Overall Layout of the Solar System All orbits but Pluto’s are close to same plane

Sense of revolution: counter-clockwise Planetary Orbits Orbits generally inclined by no more than 3.4o All planets in almost circular (elliptical) orbits around the sun, in approx. the same plane (ecliptic). Exceptions: Mercury (7o) Pluto (17.2o) Mercury Venus Mars Sense of revolution: counter-clockwise Earth Jupiter Sense of rotation: counter-clockwise (with exception of Venus, Uranus, and Pluto) Pluto Uranus Saturn Neptune (Distances and times reproduced to scale)

Terrestrial and Jovian Planets

Survey of the Solar System Relative Sizes of the Planets Assume, we reduce all bodies in the solar system so that the Earth has diameter 0.3 mm. Sun: ~ size of a small plum. Mercury, Venus, Earth, Mars: ~ size of a grain of salt. Jupiter: ~ size of an apple seed. Saturn: ~ slightly smaller than Jupiter’s “apple seed”. Pluto: ~ Speck of pepper.

Two Kinds of Planets Planets of our solar system can be divided into two very different kinds: Terrestrial (earthlike) planets: Mercury, Venus, Earth, Mars Jovian (Jupiter-like) planets: Jupiter, Saturn, Uranus, Neptune

Terrestrial Planets Four inner planets of the solar system Relatively small in size and mass (Earth is the largest and most massive) Rocky surface Surface of Venus can not be seen directly from Earth because of its dense cloud cover.

Craters on Planets’ Surfaces Craters (like on our Moon’s surface) are common throughout the Solar System. Not seen on Jovian planets because they don’t have a solid surface.

The Jovian Planets Much lower average density All have rings (not only Saturn!) Mostly gas; no solid surface

Clearing the Nebula Remains of the protostellar nebula were cleared away by: Radiation pressure of the sun Ejection by close encounters with planets Solar wind Sweeping-up of space debris by planets Surfaces of the Moon and Mercury show evidence for heavy bombardment by asteroids.

Evidence for Ongoing Planet Formation Many young stars in the Orion Nebula are surrounded by dust disks: Probably sites of planet formation right now!

Dust Disks Around Forming Stars Dust disks around some T Tauri stars can be imaged directly (HST).

Quiz Questions 1. How is the solar nebula theory supported by the motion of Solar System bodies? a. All of the planets orbit the Sun near the Sun's equatorial plane. b. All of the planets orbit in the same direction that the Sun rotates. c. Six out of seven planets rotate in the same direction as the Sun. d. Most moons orbit their planets in the same direction that the Sun rotates. e. All of the above.

Quiz Questions 2. Which of the following is NOT a property associated with terrestrial planets? a. They are located close to the Sun. b. They are small in size. c. They have low mass. d. They have low density. e. They have few moons.

Quiz Questions 3. According to the solar nebula theory, why are Jupiter and Saturn much more massive than Uranus and Neptune? a. Jupiter and Saturn formed earlier and captured nebular gas before it was cleared out. b. Jupiter and Saturn contain more high-density planet building materials. c. Uranus and Neptune have suffered more interstellar wind erosion. d. Both a and b above. e. All of the above.

Quiz Questions 4. How does the solar nebula theory account for the drastic differences between terrestrial and Jovian planets? a. The temperature of the accretion disk was high close to the Sun and low far from the Sun. b. Terrestrial planets formed closer to the Sun, and are thus made of high-density rocky materials. c. Jovian planets are large and have high-mass because they formed where both rocky and icy materials can condense. d. Jovian planets captured nebular gas as they had stronger gravity fields and are located where gases move more slowly. e. All of the above.

Quiz Questions 5. What is the difference between the processes of condensation and accretion? a. Both are processes that collect particles together. b. Condensation is the building of larger particles one atom (or molecule) at a time, whereas accretion is the sticking together of larger particles. c. Accretion is the building of larger particles one atom (or molecule) at a time, whereas condensation is the sticking together of larger particles. d. Both a and b above. e. Both a and c above.

Quiz Questions 6. Which of the following is the most likely major heat source that melted early-formed planetesimals? a. Tidal flexing. b. The impact of accreting bodies. c. The decay of long-lived unstable isotopes. d. The decay of short-lived unstable isotopes. e. The transfer of gravitational energy into thermal energy.

Quiz Questions 7. Which of the following accurately describes the differentiation process? a. High-density materials sink toward the center and low-density materials rise toward the surface of a molten body. b. Low-density materials sink toward the center and high-density materials rise toward the surface of a molten body. c. Only rocky materials can condense close to the Sun, whereas both rocky and icy materials can condense far from the Sun. d. Both rocky and icy materials can condense close to the Sun, whereas only rocky materials can condense far from the Sun. e. Small bodies stick together to form larger bodies.

Quiz Questions 8. How did the solar nebula get cleared of material? a. The radiation pressure of sunlight pushed gas particles outward. b. The intense solar wind of the youthful Sun pushed gas and dust outward. c. The planets swept up gas, dust, and small particles. d. Close gravitational encounters with Jovian planets ejected material outward. e. All of the above.

Answers 1. e 2. d 3. a 4. e 5. b 6. d 7. a 8. e